Headset for ambient noise suppression

ABSTRACT

The headset system utilizes the theory that unique sound waves can be added or subtracted without loss of information to attenuate the noise at the ear and mouthpiece, without lowering the noise level of the environment. This allows for clearer reception and transmission of communication. The system includes a directional microphone mounted on a headset/helmet, isolated from both voice and earphone output, level-setting and frequency tailoring circuits, and voltage summers and amplifiers. The microphone picks up the ambient noise, and the level-setting and frequency tailoring circuits produce a close approximation of the noise at the ear and mouthpiece (i.e., two separate matching circuits). This signal is then inverted (using an inverting buffer) to provide destructive interference.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to a headset for ambient noisesuppression.

Noisy environments, such as those found in launch facilities andaircraft cockpits, can muffle critical voice communications and causehearing loss if exposure is prolonged.

U.S. Pat. No. 2,043,416 to Lueg discloses a process of silencing soundoscillations in which the sound oscillations to be silenced arereproduced by a reproducing apparatus in the form of sound waves of anopposite phase, and then adjustable means cause the elimination of thetwo sound waves. A publication by Dr. Faux-Williams of CambridgeUniversity entitled "A Review Lecture, Anti-Sound", Sept. 8, 1984 Proc.R. Soc. London A. Vol 395, pp. 63-88 (attached) discloses the principlesby which acoustic and vibrational fields can be mimicked and cancelledby secondary sources. Also of interest are U.S. Pat. No. 2,972,018 toHawley et al, U.S. Pat. No. 4,025,724 to Davidson, Jr. et al, and U.S.Pat. Nos. 4,153,815 and 4,417,098 to Chaplin et al.

An article entitled "Noise Canceling Headset System UndergoesDevelopmental Tests" in Aviation Week & Space Technology, Nov. 24, 1986,pp. 58-59, discloses an acoustic noise canceling headset system for useby military pilots. The system was developed by the Biological AcousticsBranch of the U.S. Air Force Aerospace Medical Research Laboratory andBose Corp., and is described in a technical report AFAMRL-TR-84-008,titled Active Noise Reduction, by John Carter, available from theNational Technical Information Service (NTIS) as AD-A139741.

SUMMARY OF THE INVENTION

An object of the invention is to provide apparatus which, withoutlowering the noise level of the environment, attenuates the noise atboth the ear and mouthpiece. This allows for clearer reception andtransmission of communication.

The apparatus according to the invention comprises a headset systemwhich utilizes the theory that unique sound waves can be added orsubtracted without loss of information. The system includes adirectional microphone mounted on a headset/helmet, isolated from bothvoice and earphone output; level-setting and frequency tailoringcircuits, time delay circuits, and voltage summers and amplifiers. Themicrophone picks up the ambient noise, and the level-setting andfrequency tailoring circuits produce a close approximation of the noiseat the ear and mouthpiece (i.e., two separate matching circuits). Thesignal may then be phase-matched (to account for the spatial separationof the pickup and the earphone (or communication microphone)) by the useof all-pass filters. This signal is then inverted (using an invertingbuffer) to provide destructive interference, and added to the inputsignal before the amplification stage. This will effectively cancelnoise at the earpiece and mouthpiece.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a generic system block diagram of a headset system for noisecancellation;

FIG. 2 is a functional block diagram of a more specific circuitcorresponding to FIG. 1; and

FIG. 3 is a simplified system representation for explaining the theoryof operation.

DETAILED DESCRIPTION

FIG. 1 is a system block diagram of a headset system for noisecancellation, which utilizes the theory that unique sound waves can beadded or subtracted without loss of information. The figure is genericto account for many variances among headsets, helmets, input levels,etc. The system includes a directional noise microphone 10 mounted on aheadset/helmet 12, isolated from both voice and earphone output;level-setting and frequency and time-phase-tailoring circuits, andvoltage summers and amplifiers. FIG. 1 provides sufficient detail topermit a specific design once all parameters are defined.

The microphone 10 picks up the ambient noise, and the levelsettingfrequency and phase-tailoring circuits, which are shown in FIG. 1 as apre-amplifier 14 and two separate filter/amplifier matching circuits 16and 18, produce a close approximation of the noise at the ear andmouthpiece, respectively. The signal from the earphone filter/amplifiercircuit is then inverted using an inverting buffer 20 to providedestructive interference, and supplied as one input to a summingamplifier 24. A voice input signal on the receive line 22 of acommunication line 21 is timematched and supplied as another input ofthe summing amplifier 24, and the added signal output is supplied via anamplification stage 26 to the earpiece 28.

The signal from the filter/amplifier circuit 18 for the communicationmicrophone 38 is then inverted using an inverting buffer 30 to providedestructive interference, and supplied as one input to a summingamplifier 34. A voice input signal from the microphone 38 forcommunication is supplied via a pre-amplifier 36 and a filter unit 35 asanother input of the summing amplifier 34, and the added signal outputis supplied to the transmit line 32 of the communication line 21. Thiswill effectively cancel noise at the mouthpiece.

FIG. 2 is a functional block diagram of a more specific representativecircuit, using operational amplifiers such as type 741. Componentvalues. bias levels, and filter shapes would depend on the factorsmentioned above. The signal from the noise microphone 210 is coupled viaa capacitor 211 to the plus input of a representative pre-amplifier 214,which has its output coupled via a resistor 240 to the minus input. Theminus input is biased via a resistor 242 in series with a capacitor 244to ground, and also via a resistor 246 to ground.

The output of the pre-amplifier 214 is supplied in parallel to thesignal matching filter/amplifier units 216 for the earphone and 218 forthe communication microphone. In the earphone unit 216, a signal fromthe pre-amplifier 214 is connected to the plus input of a levelamplifier 250, which has its output connected via a resistor 252 to theminus input, the minus input being also connected via a resistor 254 toground. The output from the level set amplifier is coupled via a seriesresistor 256 and a shunt capacitor 258 to the minus input of theinverter buffer 220. The filter 256-258 illustrates the case of aheadphone shell exhibiting first-order low pass characteristics. Thefilter/amplifer unit 218 and inverter 230 for the communicationmicrophone are similar to the circuits 216 and 220, the filter incircuit 218 assuming that the head forms a first-order low pass acousticfilter.

The summing amplifier 224 has a first input connected to the output ofthe inverter buffer 220, and a second input connected to the receivepath of a voice communication line 221. The two inputs are coupled viarespective resistors 262 and 264 to the plus terminal of an op amp 260.The plus terminal is also connected via a resistor 266 to the output.The minus terminal is biased via a resistor 268 to ground. The summingamplifier 234 for the communication microphone is the same.

The output of the summing amplifier 224 is connected to the baseelectrode of a transistor amplifier, which has its collector electrodeconneced as the output to the headset earphones 228, and its emitterelectrode connected via a bias resistor 270 shunted by a capacitor 272to ground.

The summing amplifier 234 has one input connected to the circuit fromthe communication microphone 238 via a preamplifier 236, the secondinput connected to the output of the inverter buffer 230, and its outputconnected to the sending path 232 of the voice communication line 221.The circuit for the preamp 236 may be similar to that of the preamp 216.

FIG. 2 assumes that time delays caused by spatial separation isnegligible. To compensate for time delays, the filters 216 and 218 maybe all-pass filters, using an RC configuration to pass all frequencies,but provide a constant phase shift.

FIG. 3 is a simplified system representation for giving the associatedgain equations to determine the required voltage gains of thedestructive interference amplifiers. For the received signal-headsetanalyses, let

x equal the signal at the ear 329,

B_(A) be the input voice signal on line 322,

G be the system gain represented as an amplifier 323 (Vout/Vin).

N equal to the ambient noise level (db),

S_(N) be the noise microphone sensitivity represented by a block 311(Vout/db_(in)),

K₁ be the preamp/destructive interference amp total gain represented byan amplifier 320 (Vout/Vin),

the summing amplifier 324 have inputs from amplifiers 320 and 323 andoutput to a power amplifier 326,

I_(s) be the the headset/helmet isolation for the ambient noise N andthe signal input to the headset amplifier 326 (db_(out) /db_(in)), and

L be the combination gain represented by a power amplifier 326 and aloudspeaker 327, L(db_(out) /V_(in)).

To minimize noise at the ear: ##EQU1## for N to approach zero,

    S.sub.N K.sub.1 L=I.sub.s,

or

    K.sub.1 =I.sub.s /LS.sub.N.

This gain should also preserve any frequency and time characteristics(or the headphone/helmet isolation, most importantly, and alsomicrophone and speaker).

For the microphone output analysis, let

y equal the signal output to the line 332,

B_(B) be the input voice signal at the communication microphone 338,

G be the system gain represented as an amplifier 323 (Vout/Vin).

N equal to the ambient noise level.

I_(h) be the the head isolation for the ambient noise N and the signalinput to the headset amplifier 326 (db_(out) /db_(in)).

S_(c) be the comm. microphone sensitivity represented by a block 337(Vout/db_(in)).

P be the preamp gain represented by an amplifier 336 (Vout/Vin).

K₂ be the preamp/destructive interference amp total gain represented byan amplifier 330 (Vout/Vin), and

the summing amplifier 334 have inputs from amplifiers 330 and 336 andoutput to line 332.

To minimize noise at the sending output: ##EQU2## for N to approach O,

    I.sub.h S.sub.c P=S.sub.N K.sub.2,

or

    K.sub.2 =I.sub.h S.sub.c P/S.sub.N.

Again, frequency variations must be taken.

Note that this technique will not be as effective because of thefeedback from speech to the noise microphone. A cardiodpatternmicrophone would probably help alleviate this.

A concept validation test was performed using an omnidirectional dynamicmicrophone 410 (type sold under the trademark Realistic #33-985C) toperform the function of the noise pickup. The signal from thismicrophone was fed into a microphone preamp (type sold under thetrademark Realistic microphone mixer #23 583), with the output invertedin phase using an op-amp inverter and fed via a filter and a summingamplifier into an integrated audio amplifier (NAD 3020). The amplifierdrove a pair of headphones (type having the trademark K/6A). Becauseonly one channel was utilized, an earplug was placed in the subject'sleft ear, while the right channel was active. The gain of the amplifierwas set at an optimum level by ear.

The noise source was a 100 hertz square wave amplified through aloudspeaker. A square wave was chosen because of its wide spectrum. Thenoise could be characterized as "very loud ".

In spite of the limitations of this simple test, there was a markedreduction in the perceived noise level, especially in the fundamentalregion.

It is understood that certain modifications to the invention asdescribed may be made, as might occur to one with skill in the field ofthe invention, within the scope of the appended claims. Therefore, allembodiments contemplated hereunder which achieve the objects of thepresent invention have not been shown in complete detail. Otherembodiments may be developed without departing from the scope of theappended claims.

What is claimed is:
 1. A headgear system for noise cancellation whichutilizes the theory that unique sound waves can be added or subtractedwithout loss of information, comprising a directional noise microphonemounted on headgear, isolated from both voice and earphone output, areceiving destructive interference unit and a separate sendingdestructive interference unit, each of which includes signalconditioning means in tandem with phase inverting means, the receivingdestructive interference unit having an input coupled to the noisemicrophone and an output coupled to receiving summing means, the sendingdestructive interference unit having an input coupled to the noisemicrophone and an output coupled to a sending summing means, with eachsignal conditioning means comprising level-setting and frequencytailoring circuits, the receiving summing means having another inputcoupled to a line for receiving voice signals and an output coupled toearphone means of the headgear, the sending summing means having anotherinput coupled to a communication microphone of the headgear and anoutput to a line for sending voice signals.
 2. A headgear system fornoise cancellation which utilizes the theory that unique sound waves canbe added or subtracted without loss of information, comprising adirectional noise microphone mounted on headgear, isolated from bothvoice and earphone output, to pick up the ambient noise, a noisepre-amplifier coupled to the noise microphone, a receivingfilter/amplifier matching circuit and a separate sendingfilter/amplifier matching circuit, each having a level-setting amplifiercoupled to an output of the noise pre-amplifier followed by a frequencyand time-tailoring filter circuit, to produce a close approximation ofthe noise at the ear and mouthpiece, respectively;a receiving invertingbuffer coupled to an output of the receiving filter/amplifier circuitfor inverting the phase to provide destructive interference, an outputof the receiving inverting buffer being supplied as one input to areceiving summing amplifier, a voice input signal on a receive linebeing supplied as another input of the receiving summing amplifier, andthe output of the receiving summing amplifier being supplied via anamplification stage to the earphone, to effectively cancel noise at theearpiece; a sending inverting buffer coupled to an output of the sendingfilter/amplifier circuit for inverting the phase to provide destructiveinterference, an output of the sending inverting buffer being suppliedas one input to a sending summing amplifier, a voice signal from acommunication microphone in the headgear being supplied via apre-amplifier to another input of the sending summing amplifier, and theadded signal output from the sending summing amplifier being supplied toa transmit line to effectively cancel noise at the mouthpiece.